1. Field of the Invention
This invention relates to detection of trace chemicals, and more particularly to collection of particles of substances from a surface in order to provide the release, transport, and capture of the particles.
2. Description of Related Art
There exist a wide variety of chemical analysis instruments that are capable of detecting and identifying target particles of trace chemicals (“target chemicals”) once the sample of particles is transported to the instrument and subsequently vaporized. Examples include, but are not limited to, ion mobility spectrometers, mass spectrometers, gas chromatographs, surface acoustic wave sensors, cantilever beam sensors, and electron capture detectors. Similarly, there are several ways that may be utilized to transport the target particles to the instrument, some of which are incorporated within the instrument and some which may require an operator to perform the transfer. Examples include, but are not limited to, mechanically transporting a collected sample to the instrument, vacuum collection of vapor or particles, and vortex vacuum sampling.
The target particles begin by being attached to a surface by weak chemical bonds, van der Waals forces, mechanical attachment in a fibrous structure or porosity, adhesive forces, electrostatic attraction, or entrainment in a sticky material, such as grease. For some target chemicals, such as narcotics and explosives, the surface adhesion forces can be relatively strong, making the target particles difficult to remove by simple, low momentum transfer methods, such as blowing a puff of air. Removal of such strongly adhered target particles by blowing air is usually successful only for the largest and heaviest target particles that present the greatest surface area to the blowing air. In general, blowing air does not readily remove target particles of explosives or narcotics from rigid surfaces, only from flexible surfaces, such as cloth, where the fluttering motion of the material and the high porosity provides the means to mechanically dislodge the target particles. Even with cloth, the blowing air stream usually requires a very high velocity flow to have any effect and then only for the largest target particles, so the process is very inefficient.
In some cases, the process of taking a sample begins with an operator or a machine physically wiping an absorbent, often textured substance, such as chemical filter paper, onto the surface to be tested (the “target surface”). Target particles of the chemical of interest may then be transferred and concentrated on, or in, the surface texture of the absorber by the mechanical action of the wiping. This intermediate absorber is then brought to the vicinity of the detection instrument to make a measurement. The wiping method generally works reliably and efficiently but can be costly, because the media usually has to be replaced often, and a trained operator is often required. In addition, operators become tired and fail to wipe in exactly the same way each time.
There are many applications in which it may be desirable to avoid having to manually wipe a surface. These include, for example, sampling without an operator, large area sampling, remote sampling, robotic sampling, surfaces where physical contact is unacceptable, and situations in which the frequent replacement of wiping materials is not acceptable, possibly due to high cost. In these cases a better method of dislodging target particles from the surface than simply blowing air may be desirable.
A non-contact apparatus for cleaning a silicon semiconductor wafer is known. See for example, U.S. Pat. No. 5,931,721 by Rose et al., “Aerosol Surface Processing”. Aerosols of liquids or solids are used to abrade contamination particles and films from a silicon wafer surface without damaging the semiconductor devices previously constructed on the wafer. This apparatus is for cleaning a wafer, and no effort is made to capture the resultant particles for chemical analysis. In addition, other cleansing features, such as rinsing the wafer, are required in addition to the aerosol surface processing.